If Focal Point attempts to load an image and the image is not present, this can lead to an InvalidArgumentException being thrown, which breaks site display. Ideally this should just lead to the image not being rendered (as would normally happen for a missing image).
There may be special circumstances that lead to this being an issue on the site I've seen it on. It looks like the image is coming via shortcut module (?), and this happens even when viewing the site's admin interface (which shouldn't be hitting the custom theme, though that's visible below).
Observing this behaviour after clearing cache, and on a dev environment which lacks a local copy of the site media (hence $image->isValid() is not true).
Error displayed is:
The website encountered an unexpected error. Please try again later.Symfony\Component\Validator\Exception\InvalidArgumentException: The file with id = $fid is not an image. in Drupal\focal_point\Controller\FocalPointPreviewController->access() (line 117 of modules/contrib/focal_point/src/Controller/FocalPointPreviewController.php). call_user_func_array(Array, Array) (Line: 68)
Drupal\Core\Access\CustomAccessCheck->access(Object, Object, Object)
call_user_func_array(Array, Array) (Line: 159)
Drupal\Core\Access\AccessManager->performCheck('access_check.custom', Object) (Line: 135)
Drupal\Core\Access\AccessManager->check(Object, Object, NULL, ) (Line: 92)
Drupal\Core\Access\AccessManager->checkNamedRoute('focal_point.preview', Array, NULL) (Line: 809)
Drupal\Core\Url->access() (Line: 269)
shortcut_renderable_links() (Line: 392)
shortcut_toolbar()
call_user_func_array('shortcut_toolbar', Array) (Line: 402)
Drupal\Core\Extension\ModuleHandler->invokeAll('toolbar') (Line: 81)
Drupal\toolbar\Element\Toolbar::preRenderToolbar(Array)
call_user_func(Array, Array) (Line: 378)
Drupal\Core\Render\Renderer->doRender(Array) (Line: 450)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 195)
Drupal\Core\Render\Renderer->render(Array) (Line: 507)
Drupal\Core\Template\TwigExtension->escapeFilter(Object, Array, 'html', NULL, 1) (Line: 86)
__TwigTemplate_370319f7193408b12a6a6181dd4824874cf4f39e1bfbacc823a86b5ab4b1722f->doDisplay(Array, Array) (Line: 432)
Twig_Template->displayWithErrorHandling(Array, Array) (Line: 403)
Twig_Template->display(Array) (Line: 411)
Twig_Template->render(Array) (Line: 64)
twig_render_template('themes/custom/example/templates/system/html.html.twig', Array) (Line: 384)
Drupal\Core\Theme\ThemeManager->render('html', Array) (Line: 437)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 195)
Drupal\Core\Render\Renderer->render(Array) (Line: 147)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 582)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 148)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object, Object) (Line: 90)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object, 'kernel.view', Object)
call_user_func(Array, Object, 'kernel.view', Object) (Line: 111)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.view', Object) (Line: 156)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 99)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 78)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 666)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)